[Fix, Feat, Test] (MG_Impl, MG_Test): a specific compressed internalformat now tags the level it defines, and the compressed sub-image entry points stopped being stubs

MobileGL/MG_Impl/GLImpl/Exporting/Definitions.cpp

@@ -1061,7 +1061,7 @@ DECLARE_GL_FUNCTION_HEAD(void, TextureSubImage1D, GLuint texture, GLint level, G
 DECLARE_GL_FUNCTION_HEAD(void, TextureSubImage2D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLsizei width, GLsizei height, GLenum format, GLenum type, const void* pixels) DECLARE_GL_FUNCTION_END_NO_RETURN(void, TextureSubImage2D, texture, level, xoffset, yoffset, width, height, format, type, pixels)
 DECLARE_GL_FUNCTION_HEAD(void, TextureSubImage3D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLsizei width, GLsizei height, GLsizei depth, GLenum format, GLenum type, const void* pixels) DECLARE_GL_FUNCTION_END_NO_RETURN(void, TextureSubImage3D, texture, level, xoffset, yoffset, zoffset, width, height, depth, format, type, pixels)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureSubImage1D, GLuint texture, GLint level, GLint xoffset, GLsizei width, GLenum format, GLsizei imageSize, const void* data) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureSubImage1D, texture, level, xoffset, width, format, imageSize, data)
-DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureSubImage2D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLsizei width, GLsizei height, GLenum format, GLsizei imageSize, const void* data) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureSubImage2D, texture, level, xoffset, yoffset, width, height, format, imageSize, data)
+DECLARE_GL_FUNCTION_HEAD(void, CompressedTextureSubImage2D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLsizei width, GLsizei height, GLenum format, GLsizei imageSize, const void* data) DECLARE_GL_FUNCTION_END_NO_RETURN(void, CompressedTextureSubImage2D, texture, level, xoffset, yoffset, width, height, format, imageSize, data)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureSubImage3D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLsizei width, GLsizei height, GLsizei depth, GLenum format, GLsizei imageSize, const void* data) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureSubImage3D, texture, level, xoffset, yoffset, zoffset, width, height, depth, format, imageSize, data)
 DECLARE_GL_FUNCTION_HEAD(void, CopyTextureSubImage1D, GLuint texture, GLint level, GLint xoffset, GLint x, GLint y, GLsizei width) DECLARE_GL_FUNCTION_END_NO_RETURN(void, CopyTextureSubImage1D, texture, level, xoffset, x, y, width)
 DECLARE_GL_FUNCTION_HEAD(void, CopyTextureSubImage2D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLint x, GLint y, GLsizei width, GLsizei height) DECLARE_GL_FUNCTION_END_NO_RETURN(void, CopyTextureSubImage2D, texture, level, xoffset, yoffset, x, y, width, height)
@@ -1849,7 +1849,7 @@ DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureImage3DEXT, GLuint texture,
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureImage2DEXT, GLuint texture, GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height, GLint border, GLsizei imageSize, const void* bits) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureImage2DEXT, texture, target, level, internalformat, width, height, border, imageSize, bits)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureImage1DEXT, GLuint texture, GLenum target, GLint level, GLenum internalformat, GLsizei width, GLint border, GLsizei imageSize, const void* bits) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureImage1DEXT, texture, target, level, internalformat, width, border, imageSize, bits)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureSubImage3DEXT, GLuint texture, GLenum target, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLsizei width, GLsizei height, GLsizei depth, GLenum format, GLsizei imageSize, const void* bits) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureSubImage3DEXT, texture, target, level, xoffset, yoffset, zoffset, width, height, depth, format, imageSize, bits)
-DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureSubImage2DEXT, GLuint texture, GLenum target, GLint level, GLint xoffset, GLint yoffset, GLsizei width, GLsizei height, GLenum format, GLsizei imageSize, const void* bits) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureSubImage2DEXT, texture, target, level, xoffset, yoffset, width, height, format, imageSize, bits)
+DECLARE_GL_FUNCTION_HEAD(void, CompressedTextureSubImage2DEXT, GLuint texture, GLenum target, GLint level, GLint xoffset, GLint yoffset, GLsizei width, GLsizei height, GLenum format, GLsizei imageSize, const void* bits) DECLARE_GL_FUNCTION_END_NO_RETURN(void, CompressedTextureSubImage2D, texture, level, xoffset, yoffset, width, height, format, imageSize, bits)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureSubImage1DEXT, GLuint texture, GLenum target, GLint level, GLint xoffset, GLsizei width, GLenum format, GLsizei imageSize, const void* bits) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureSubImage1DEXT, texture, target, level, xoffset, width, format, imageSize, bits)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, GetCompressedTextureImageEXT, GLuint texture, GLenum target, GLint lod, void* img) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, GetCompressedTextureImageEXT, texture, target, lod, img)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedMultiTexImage3DEXT, GLenum texunit, GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height, GLsizei depth, GLint border, GLsizei imageSize, const void* bits) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedMultiTexImage3DEXT, texunit, target, level, internalformat, width, height, depth, border, imageSize, bits)

MobileGL/MG_Impl/GLImpl/Texture/GL_Texture.cpp

@@ -2238,6 +2238,23 @@ namespace MobileGL::MG_Impl::GLImpl {
             DiscardMipmapChainOnBaseRespecification(textureMipmapObject, textureUploadTarget, level);
             textureMipmapObject->AllocateStorage(textureUploadTarget, level,
                                                  {{width, height, 1}, internalBytes});
+            // GL 4.6 core 8.5: a SPECIFIC compressed internalformat (unlike a generic
+            // GL_COMPRESSED_* one, where the implementation is free to choose) commits the
+            // level to that format - GL_TEXTURE_COMPRESSED must then answer true for it and
+            // GL_TEXTURE_INTERNAL_FORMAT must report it, which is how an application asks for
+            // the size to hand glCompressedTexSubImage2D afterwards. Only the tag and the size
+            // are recorded: there is no BC/ETC codec here, so the texel shadow keeps the
+            // uncompressed storage this format resolved to (which is also what lets the level
+            // sample as the application's texels), and the compressed image the tag describes
+            // is zero-filled - the one reproducible answer glGetCompressedTexImage can give for
+            // an image nothing ever compressed. AllocateStorage above clears the tag, so this
+            // has to follow it.
+            const auto compressedInfo = MG_Util::GetCompressedFormatInfo(static_cast<GLenum>(internalformat));
+            if (compressedInfo.blockWidth != 0) {
+                textureMipmapObject->SetMipmapCompressedImage(
+                    textureUploadTarget, level, static_cast<GLenum>(internalformat), nullptr,
+                    MG_Util::CalculateCompressedTextureImageSize(compressedInfo, {width, height, 1}));
+            }
         }
 
         if (!originalPixels) {
@@ -2965,9 +2982,9 @@ namespace MobileGL::MG_Impl::GLImpl {
         case GL_TEXTURE_INTERNAL_FORMAT:
             if (params) {
                 // A level stored compressed must report the token it was given, not the
-                // uncompressed format backing it (GL 4.6 core 8.11). Only glCompressedTexImage* sets
-                // that tag, so every level created by glTexImage*D - including one given a compressed
-                // internalformat - still answers with its resolved storage format.
+                // uncompressed format backing it (GL 4.6 core 8.11). glCompressedTexImage2D sets
+                // that tag, and so does a glTexImage2D given a SPECIFIC compressed internalformat;
+                // every other level answers with its resolved storage format.
                 const GLenum compressedFormat = GetCompressedLevelFormat(textureObject, textureUploadTarget, level);
                 *params = (compressedFormat != GL_NONE)
                               ? (GLint)compressedFormat
@@ -3103,9 +3120,9 @@ namespace MobileGL::MG_Impl::GLImpl {
         case GL_TEXTURE_INTERNAL_FORMAT:
             if (params) {
                 // A level stored compressed must report the token it was given, not the
-                // uncompressed format backing it (GL 4.6 core 8.11). Only glCompressedTexImage* sets
-                // that tag, so every level created by glTexImage*D - including one given a compressed
-                // internalformat - still answers with its resolved storage format.
+                // uncompressed format backing it (GL 4.6 core 8.11). glCompressedTexImage2D sets
+                // that tag, and so does a glTexImage2D given a SPECIFIC compressed internalformat;
+                // every other level answers with its resolved storage format.
                 const GLenum compressedFormat = GetCompressedLevelFormat(textureObject, textureUploadTarget, level);
                 *params = (GLfloat)((compressedFormat != GL_NONE)
                                         ? compressedFormat
@@ -3468,10 +3485,170 @@ namespace MobileGL::MG_Impl::GLImpl {
         RecordUnsupportedCompressedFormat(__func__);
     }
 
+    // Replaces a block-aligned rectangle of the compressed image glCompressedTexImage2D (or a
+    // compressed glTexImage2D) shadowed for this level. Same deviation as the image call it
+    // patches: the uncompressed texel shadow beside it is NOT touched, because there is no
+    // BC/ETC codec here to decode the incoming blocks with - so what changes is the image
+    // glGetCompressedTexImage hands back, not what the level samples as. Marking the texels
+    // dirty would therefore only re-upload bytes that did not change.
     void CompressedTexSubImage2D_State(GLenum target, GLint level, GLint xoffset, GLint yoffset, GLsizei width,
                                        GLsizei height, GLenum format, GLsizei imageSize, const void* data) {
-        // TODO: implement compressed upload - see CompressedTexImage2D_State.
-        RecordUnsupportedCompressedFormat(__func__);
+        // ======================= Converting ================================
+        const auto textureUploadTarget = MG_Util::ConvertGLEnumToTextureUploadTarget(target);
+        const auto textureTarget = MG_Util::ConvertGLEnumToTextureTarget(target);
+        // Zero block width doubles as "format is not a specific compressed format", the
+        // INVALID_ENUM case - one lookup answers both questions.
+        const auto compressedInfo = MG_Util::GetCompressedFormatInfo(format);
+
+        // ===================== Error Checking ==============================
+        if (!TextureImpl::ValidateTextureUploadTarget(textureUploadTarget)) return;
+        // A proxy holds no image to modify; only the glTexImage*/glCompressedTexImage* pair
+        // accepts one.
+        if (TextureImpl::IsProxyTextureTarget(textureUploadTarget)) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidEnum,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             "A proxy target has no texture image to modify."));
+            return;
+        }
+        if (!TextureImpl::ValidateTextureLevelNumber(level)) return;
+        if (!TextureImpl::ValidateTextureLevelWithUploadTarget(textureUploadTarget, level)) return;
+        if (width < 0 || height < 0) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidValue,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__, "width and height must be non-negative."));
+            return;
+        }
+        if (compressedInfo.blockWidth == 0) {
+            RecordUnsupportedCompressedFormat(__func__);
+            return;
+        }
+
+        auto& textureObject = GetTextureObjectByTarget(textureUploadTarget, textureTarget);
+        if (!TextureImpl::ValidateTextureObject(textureObject)) return;
+        auto* textureMipmapObject = MG_State::GLState::AsMipmapTexture(textureObject.get());
+        if (textureMipmapObject == nullptr) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidOperation,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__, "Texture storage is not mipmap-backed."));
+            return;
+        }
+        // GL 4.6 core 8.7: INVALID_OPERATION unless the image being modified is stored in
+        // exactly this compressed format. That is also what makes the block arithmetic below
+        // sound - the level's grid is measured with THIS format's block size.
+        const GLenum levelFormat =
+            textureMipmapObject->GetMipmapCompressedFormat(textureUploadTarget, static_cast<Uint>(level));
+        if (levelFormat != format) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidOperation,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             "format does not match the internal format of the texture image."));
+            return;
+        }
+
+        const IntVec3 levelSize = textureMipmapObject->GetMipmapTexelSize(textureUploadTarget, static_cast<Uint>(level));
+        if (xoffset < 0 || yoffset < 0 || xoffset + width > levelSize.x() || yoffset + height > levelSize.y()) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidValue,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             "The replaced region does not lie within the texture image."));
+            return;
+        }
+        // GL 4.6 core 8.7 for block-based formats: the region must start on a block boundary
+        // and must either be a whole number of blocks wide/high or run to the image's edge.
+        const Int blockWidth = static_cast<Int>(compressedInfo.blockWidth);
+        const Int blockHeight = static_cast<Int>(compressedInfo.blockHeight);
+        const Bool alignedX = (xoffset % blockWidth == 0) &&
+                              (width % blockWidth == 0 || xoffset + width == levelSize.x());
+        const Bool alignedY = (yoffset % blockHeight == 0) &&
+                              (height % blockHeight == 0 || yoffset + height == levelSize.y());
+        if (!alignedX || !alignedY) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidOperation,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             "The replaced region is not aligned to the format's compressed blocks."));
+            return;
+        }
+        // Exactly the size the format and dimensions imply, which is also what keeps the copy
+        // below in bounds.
+        const SizeT expectedImageSize =
+            MG_Util::CalculateCompressedTextureImageSize(compressedInfo, {width, height, 1});
+        if (imageSize < 0 || static_cast<SizeT>(imageSize) != expectedImageSize) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidValue,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             "imageSize does not match the compressed image size."));
+            return;
+        }
+
+        // ======================= Processing ================================
+        const void* compressedBytes = data;
+        const auto& pixelUnpackBufferObject =
+            MG_State::pGLContext->GetBufferBindingSlot(BufferTarget::PixelUnpack).GetBoundObject();
+        if (pixelUnpackBufferObject) {
+            if (pixelUnpackBufferObject->IsMapped()) {
+                MG_State::pGLContext->RecordError(
+                    ErrorCode::InvalidOperation,
+                    MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                                 "Pixel unpack buffer is currently mapped."));
+                return;
+            }
+            const SizeT offset = reinterpret_cast<SizeT>(data);
+            const SizeT bufferSize = pixelUnpackBufferObject->GetSize();
+            if (offset > bufferSize || expectedImageSize > bufferSize - offset) {
+                MG_State::pGLContext->RecordError(
+                    ErrorCode::InvalidOperation,
+                    MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                                 "Unpacking would read past the end of the pixel unpack buffer."));
+                return;
+            }
+            compressedBytes = reinterpret_cast<const char*>(pixelUnpackBufferObject->MappedData()) + offset;
+        }
+        if (expectedImageSize == 0) return; // a zero-sized region is a legal no-op
+        if (compressedBytes == nullptr) {
+            // No unpack buffer and a null client pointer: there is nothing to read. GL leaves
+            // this undefined rather than erroring, and dereferencing it is the one answer that
+            // is never acceptable.
+            MGLOG_D("%s: null data with no pixel unpack buffer bound, nothing to replace", __func__);
+            return;
+        }
+
+        // The level's compressed image is stored as one blob, so the rectangle is patched into
+        // a copy of it and the whole thing handed back. Compressed sub-image uploads are not a
+        // hot path, and this keeps the storage layer's compressed API to the two calls it has.
+        const SizeT blobSize =
+            textureMipmapObject->GetMipmapCompressedByteSize(textureUploadTarget, static_cast<Uint>(level));
+        const void* existing =
+            textureMipmapObject->MapMipmapCompressedImage(textureUploadTarget, static_cast<Uint>(level));
+        if (blobSize == 0 || existing == nullptr) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidOperation,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             "The texture level holds no compressed image to modify."));
+            return;
+        }
+        Vector<Uint8> blob(blobSize);
+        Memcpy(blob.data(), existing, blobSize);
+
+        const SizeT blockByteSize = compressedInfo.blockByteSize;
+        const SizeT levelBlocksX = (static_cast<SizeT>(levelSize.x()) + compressedInfo.blockWidth - 1) /
+                                   compressedInfo.blockWidth;
+        const SizeT levelRowBytes = levelBlocksX * blockByteSize;
+        const SizeT regionBlocksX = (static_cast<SizeT>(width) + compressedInfo.blockWidth - 1) /
+                                    compressedInfo.blockWidth;
+        const SizeT regionBlocksY = (static_cast<SizeT>(height) + compressedInfo.blockHeight - 1) /
+                                    compressedInfo.blockHeight;
+        const SizeT firstBlockX = static_cast<SizeT>(xoffset) / compressedInfo.blockWidth;
+        const SizeT firstBlockY = static_cast<SizeT>(yoffset) / compressedInfo.blockHeight;
+        const SizeT regionRowBytes = regionBlocksX * blockByteSize;
+        const auto* source = static_cast<const Uint8*>(compressedBytes);
+        for (SizeT row = 0; row < regionBlocksY; ++row) {
+            const SizeT destOffset = (firstBlockY + row) * levelRowBytes + firstBlockX * blockByteSize;
+            if (destOffset + regionRowBytes > blobSize) break; // a level whose blob predates its size
+            Memcpy(blob.data() + destOffset, source + row * regionRowBytes, regionRowBytes);
+        }
+        textureMipmapObject->SetMipmapCompressedImage(textureUploadTarget, static_cast<Uint>(level), format,
+                                                      blob.data(), blobSize);
     }
 
     void CompressedTexSubImage1D_State(GLenum target, GLint level, GLint xoffset, GLsizei width, GLenum format,
@@ -4472,6 +4649,14 @@ namespace MobileGL::MG_Impl::GLImpl {
         free(processedPixels);
     }
 
+    void CompressedTextureSubImage2D(GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLsizei width,
+                                     GLsizei height, GLenum format, GLsizei imageSize, const void* data) {
+        auto textureObject = GetTextureObjectByName(texture, __func__);
+        WithTemporarilyBoundNamedTexture(textureObject, [&](GLenum target) {
+            CompressedTexSubImage2D_State(target, level, xoffset, yoffset, width, height, format, imageSize, data);
+        });
+    }
+
     void TextureSubImage3D(GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLsizei width,
                            GLsizei height, GLsizei depth, GLenum format, GLenum type, const void* pixels) {
         auto textureObject = GetTextureObjectByName(texture, __func__);

MobileGL/MG_Impl/GLImpl/Texture/GL_Texture.h

@@ -37,6 +37,8 @@ namespace MobileGL::MG_Impl::GLImpl {
                            GLenum format, GLenum type, const void* pixels);
     void TextureSubImage3D(GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLsizei width,
                            GLsizei height, GLsizei depth, GLenum format, GLenum type, const void* pixels);
+    void CompressedTextureSubImage2D(GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLsizei width,
+                                     GLsizei height, GLenum format, GLsizei imageSize, const void* data);
     void TextureParameterf(GLuint texture, GLenum pname, GLfloat param);
     void TextureParameterfv(GLuint texture, GLenum pname, const GLfloat* params);
     void TextureParameteri(GLuint texture, GLenum pname, GLint param);

MobileGL/MG_Test/Texture/TextureTest.cpp

@@ -16,6 +16,7 @@
 #include <MG_Backend/BackendObjects.h>
 #include <MG_Backend/DirectGLES/Managers.h>
 #include <MG_Backend/DirectGLES/Utils.h>
+#include <MG_Impl/GLImpl/Buffer/GL_Buffer.h>
 #include <MG_Impl/GLImpl/Framebuffer/GL_Framebuffer.h>
 #include <MG_Impl/GLImpl/Getter/GL_Getter.h>
 #include <MG_Impl/GLImpl/RenderState/GL_RenderState.h>
@@ -1572,6 +1573,238 @@ TEST_F(TextureTest, CompressedInternalFormatsResolveToTheirUncompressedStorage)
     }
 }
 
+// Resolving to uncompressed storage is a storage decision, not a licence to answer the level
+// queries as if the application had asked for an uncompressed format. GL 4.6 core 8.5 lets the
+// implementation choose for the GENERIC formats (GL_COMPRESSED_RED and friends), but a SPECIFIC
+// one commits the level: GL_TEXTURE_COMPRESSED is true, GL_TEXTURE_INTERNAL_FORMAT is the token
+// that was passed, and GL_TEXTURE_COMPRESSED_IMAGE_SIZE answers instead of erroring - which is
+// exactly the three-query sequence KHR-GL44.buffer_storage.map_persistent_texture opens with to
+// size the image it then uploads through glCompressedTexSubImage2D.
+TEST_F(TextureTest, ASpecificCompressedInternalFormatTagsTheLevelCompressed) {
+    GLuint texture = 0;
+    MG_Impl::GLImpl::GenTextures(1, &texture);
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, texture);
+    MG_Impl::GLImpl::TexImage2D(GL_TEXTURE_2D, 0, GL_COMPRESSED_RED_RGTC1, 8, 8, 0, GL_RED, GL_UNSIGNED_BYTE, nullptr);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    GLint compressed = GL_FALSE;
+    MG_Impl::GLImpl::GetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_COMPRESSED, &compressed);
+    EXPECT_EQ(compressed, GL_TRUE);
+
+    GLint internalFormat = 0;
+    MG_Impl::GLImpl::GetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_INTERNAL_FORMAT, &internalFormat);
+    EXPECT_EQ(internalFormat, static_cast<GLint>(GL_COMPRESSED_RED_RGTC1));
+
+    // 8x8 in 4x4 blocks of 8 bytes each.
+    GLint imageSize = 0;
+    MG_Impl::GLImpl::GetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &imageSize);
+    EXPECT_EQ(imageSize, 32);
+    EXPECT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    // The texel shadow behind the tag still carries the uncompressed storage the format resolves
+    // to - which is what lets the level sample, and what every size computation downstream
+    // divides by.
+    const auto textureObject = MG_State::pGLContext->GetTextureObject(texture);
+    ASSERT_NE(textureObject, nullptr);
+    EXPECT_EQ(textureObject->GetFormat(), TextureInternalFormat::R8);
+}
+
+// The negative control for the case above, and the reason it cannot simply tag every
+// GL_COMPRESSED_* token: for a generic format the implementation's choice IS the answer, and
+// MobileGL chooses uncompressed - so the level is not compressed and the size query is the
+// INVALID_OPERATION GL 4.6 core 8.11 prescribes for an uncompressed image.
+TEST_F(TextureTest, AGenericCompressedInternalFormatLeavesTheLevelUncompressed) {
+    GLuint texture = 0;
+    MG_Impl::GLImpl::GenTextures(1, &texture);
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, texture);
+    MG_Impl::GLImpl::TexImage2D(GL_TEXTURE_2D, 0, GL_COMPRESSED_RED, 8, 8, 0, GL_RED, GL_UNSIGNED_BYTE, nullptr);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    GLint compressed = GL_TRUE;
+    MG_Impl::GLImpl::GetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_COMPRESSED, &compressed);
+    EXPECT_EQ(compressed, GL_FALSE);
+
+    GLint internalFormat = 0;
+    MG_Impl::GLImpl::GetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_INTERNAL_FORMAT, &internalFormat);
+    EXPECT_EQ(internalFormat, static_cast<GLint>(GL_R8));
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    GLint imageSize = 0;
+    MG_Impl::GLImpl::GetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &imageSize);
+    ExpectSingleGlError(GL_INVALID_OPERATION);
+}
+
+// A plain glTexImage2D over a level that was tagged compressed has to un-tag it, the same way it
+// does for a level a glCompressedTexImage2D shadowed - otherwise the size query would keep
+// answering for an image that no longer exists.
+TEST_F(TextureTest, AnUncompressedRespecificationClearsTheCompressedTag) {
+    GLuint texture = 0;
+    MG_Impl::GLImpl::GenTextures(1, &texture);
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, texture);
+    MG_Impl::GLImpl::TexImage2D(GL_TEXTURE_2D, 0, GL_COMPRESSED_RED_RGTC1, 8, 8, 0, GL_RED, GL_UNSIGNED_BYTE, nullptr);
+    MG_Impl::GLImpl::TexImage2D(GL_TEXTURE_2D, 0, GL_R8, 8, 8, 0, GL_RED, GL_UNSIGNED_BYTE, nullptr);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    GLint compressed = GL_TRUE;
+    MG_Impl::GLImpl::GetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_COMPRESSED, &compressed);
+    EXPECT_EQ(compressed, GL_FALSE);
+    EXPECT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+}
+
+namespace {
+    // 8x8 RGTC1: 2x2 blocks of 8 bytes, so the stored image is 32 bytes and one block row is 16.
+    constexpr GLsizei kRgtc1Size8x8 = 32;
+
+    GLuint MakeCompressedRgtc1Texture8x8() {
+        GLuint texture = 0;
+        MG_Impl::GLImpl::GenTextures(1, &texture);
+        MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, texture);
+        MG_Impl::GLImpl::CompressedTexImage2D(GL_TEXTURE_2D, 0, GL_COMPRESSED_RED_RGTC1, 8, 8, 0, kRgtc1Size8x8,
+                                              nullptr);
+        return texture;
+    }
+} // namespace
+
+// glCompressedTexSubImage2D was a stub that answered GL_INVALID_ENUM to every call. It replaces a
+// block-aligned rectangle of the stored image, and the arithmetic that places the incoming blocks
+// is what the partial write below pins: a full-width write would pass with the rows concatenated
+// in either order.
+TEST_F(TextureTest, CompressedTexSubImage2DReplacesTheStoredBlocks) {
+    const GLuint texture = MakeCompressedRgtc1Texture8x8();
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    Uint8 whole[kRgtc1Size8x8];
+    for (Int i = 0; i < kRgtc1Size8x8; ++i) whole[i] = static_cast<Uint8>(i + 1);
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 8, 8, GL_COMPRESSED_RED_RGTC1, kRgtc1Size8x8,
+                                             whole);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    Uint8 stored[kRgtc1Size8x8] = {};
+    MG_Impl::GLImpl::GetCompressedTexImage(GL_TEXTURE_2D, 0, stored);
+    EXPECT_EQ(std::memcmp(stored, whole, sizeof(whole)), 0);
+
+    // The right-hand block column only: one block wide, two block rows high. Its two blocks land
+    // at byte 8 and byte 24, not at bytes 0 and 8.
+    const Uint8 column[16] = {0xA0, 0xA1, 0xA2, 0xA3, 0xA4, 0xA5, 0xA6, 0xA7,
+                              0xB0, 0xB1, 0xB2, 0xB3, 0xB4, 0xB5, 0xB6, 0xB7};
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 4, 0, 4, 8, GL_COMPRESSED_RED_RGTC1,
+                                             static_cast<GLsizei>(sizeof(column)), column);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    Uint8 expected[kRgtc1Size8x8];
+    std::memcpy(expected, whole, sizeof(expected));
+    std::memcpy(expected + 8, column, 8);
+    std::memcpy(expected + 24, column + 8, 8);
+
+    std::memset(stored, 0, sizeof(stored));
+    MG_Impl::GLImpl::GetCompressedTexImage(GL_TEXTURE_2D, 0, stored);
+    EXPECT_EQ(std::memcmp(stored, expected, sizeof(expected)), 0);
+    EXPECT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+}
+
+// The same call sourcing its blocks from a buffer bound to GL_PIXEL_UNPACK_BUFFER, where `data` is
+// an offset into that buffer rather than a client pointer - which is the form
+// KHR-GL44.buffer_storage.map_persistent_texture uses for every one of its operations.
+TEST_F(TextureTest, CompressedTexSubImage2DUnpacksFromAPixelUnpackBuffer) {
+    Uint8 source[256];
+    for (Int i = 0; i < 256; ++i) source[i] = static_cast<Uint8>(i);
+    GLuint buffer = 0;
+    MG_Impl::GLImpl::GenBuffers(1, &buffer);
+    MG_Impl::GLImpl::BindBuffer(GL_PIXEL_UNPACK_BUFFER, buffer);
+    MG_Impl::GLImpl::BufferData(GL_PIXEL_UNPACK_BUFFER, sizeof(source), source, GL_STATIC_DRAW);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    const GLuint texture = MakeCompressedRgtc1Texture8x8();
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 8, 8, GL_COMPRESSED_RED_RGTC1, kRgtc1Size8x8,
+                                             reinterpret_cast<const void*>(static_cast<SizeT>(64)));
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    Uint8 stored[kRgtc1Size8x8] = {};
+    MG_Impl::GLImpl::GetCompressedTexImage(GL_TEXTURE_2D, 0, stored);
+    EXPECT_EQ(std::memcmp(stored, source + 64, sizeof(stored)), 0);
+
+    // Reading past the end of the buffer is the unpack-buffer error, not a read out of bounds.
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 8, 8, GL_COMPRESSED_RED_RGTC1, kRgtc1Size8x8,
+                                             reinterpret_cast<const void*>(static_cast<SizeT>(sizeof(source) - 8)));
+    ExpectSingleGlError(GL_INVALID_OPERATION);
+
+    MG_Impl::GLImpl::BindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
+    (void)texture;
+}
+
+// glCompressedTextureSubImage2D was an exported no-op that raised no error at all, so an
+// application could not tell the write had not happened. It must reach the NAMED texture and leave
+// the binding it borrowed exactly as it found it.
+TEST_F(TextureTest, CompressedTextureSubImage2DModifiesTheNamedTextureOnly) {
+    const GLuint bound = MakeCompressedRgtc1Texture8x8();
+    Uint8 boundImage[kRgtc1Size8x8];
+    for (Int i = 0; i < kRgtc1Size8x8; ++i) boundImage[i] = 0x11;
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 8, 8, GL_COMPRESSED_RED_RGTC1, kRgtc1Size8x8,
+                                             boundImage);
+
+    const GLuint named = MakeCompressedRgtc1Texture8x8();
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, bound); // `named` is NOT the bound texture
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    Uint8 namedImage[kRgtc1Size8x8];
+    for (Int i = 0; i < kRgtc1Size8x8; ++i) namedImage[i] = 0x22;
+    MG_Impl::GLImpl::CompressedTextureSubImage2D(named, 0, 0, 0, 8, 8, GL_COMPRESSED_RED_RGTC1, kRgtc1Size8x8,
+                                                 namedImage);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    // The borrowed binding is back, and it kept its own image.
+    Uint8 stored[kRgtc1Size8x8] = {};
+    MG_Impl::GLImpl::GetCompressedTexImage(GL_TEXTURE_2D, 0, stored);
+    EXPECT_EQ(std::memcmp(stored, boundImage, sizeof(stored)), 0);
+
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, named);
+    std::memset(stored, 0, sizeof(stored));
+    MG_Impl::GLImpl::GetCompressedTexImage(GL_TEXTURE_2D, 0, stored);
+    EXPECT_EQ(std::memcmp(stored, namedImage, sizeof(stored)), 0);
+    EXPECT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+}
+
+TEST_F(TextureTest, CompressedTexSubImage2DRejectsTheRegionsGLForbids) {
+    const GLuint texture = MakeCompressedRgtc1Texture8x8();
+    Uint8 blocks[kRgtc1Size8x8] = {};
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    // A format that is not the one the image is stored in.
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 8, 8, GL_COMPRESSED_RG_RGTC2, 64, blocks);
+    ExpectSingleGlError(GL_INVALID_OPERATION);
+
+    // A start that is not on a block boundary.
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 2, 0, 4, 8, GL_COMPRESSED_RED_RGTC1, 16, blocks);
+    ExpectSingleGlError(GL_INVALID_OPERATION);
+
+    // A width that is neither a whole number of blocks nor a run to the image's edge.
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 2, 8, GL_COMPRESSED_RED_RGTC1, 16, blocks);
+    ExpectSingleGlError(GL_INVALID_OPERATION);
+
+    // A region that runs off the image.
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 4, 0, 8, 8, GL_COMPRESSED_RED_RGTC1, 32, blocks);
+    ExpectSingleGlError(GL_INVALID_VALUE);
+
+    // An imageSize that does not match the region.
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 8, 8, GL_COMPRESSED_RED_RGTC1, 16, blocks);
+    ExpectSingleGlError(GL_INVALID_VALUE);
+
+    // A format with no defined block layout here.
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 8, 8, GL_RGBA8, 32, blocks);
+    ExpectSingleGlError(GL_INVALID_ENUM);
+
+    // An uncompressed image has nothing for it to replace.
+    GLuint plain = 0;
+    MG_Impl::GLImpl::GenTextures(1, &plain);
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, plain);
+    MG_Impl::GLImpl::TexImage2D(GL_TEXTURE_2D, 0, GL_R8, 8, 8, 0, GL_RED, GL_UNSIGNED_BYTE, nullptr);
+    ASSERT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+    MG_Impl::GLImpl::CompressedTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 8, 8, GL_COMPRESSED_RED_RGTC1, kRgtc1Size8x8,
+                                             blocks);
+    ExpectSingleGlError(GL_INVALID_OPERATION);
+    (void)texture;
+}
+
 // RGTC compresses 4x4 blocks of a 2D image and has no 3D form, so glTexImage3D must reject it even
 // though the same enum is accepted on a 2D target. The generic compressed formats carry no such
 // restriction and stay legal in 3D.